1. Field of the Invention
This invention relates generally to internally stiffened shell structures and, more particularly, shell structured aerodynamic blades and vanes and a method for fabricating the same.
2. Prior Art
Power conversion equipment which utilizes the change in velocity and direction of gas flow requires the use of rotating blades and stationary vanes. For airborne equipment it is necessary that structural concepts, yielding maximum strength and stiffness to weight ratios, be utilized to achieve the required power to weight efficiency. In all cases, it is desirable to minimize the weight of the rotating blades. Blades and vanes are thin aerodynamic shapes with varying degrees of camber, twist, and thickness as a function of the gas flow requirements. The gas pressure and flow characteristics impose bending strength and stiffness, torsional strength and stiffness, and shear strength and stiffness requirements upon the structural configuration of the blade or vane.
It is known that the lightest weight structures are achieved by utilizing a material having a high strength to weight ratio in conjunction with a structural configuration which places this material at the periphery of the structure, i.e., a hollow section. However, when using a hollow section with thin material, the material becomes unstable in compression and shear buckling modes. The use of internal stiffening material to stabilize the thin facing skins of hollow sections has been developed and proved in service for many years. While such internally stiffened shell structures are known in the prior art, they have been used in substantially planar configurations and large shells, and heretofore it has not been known how to fabricate them in the intricate shapes and geometric forms required by many applications, such as, for example, aerodynamic blades and vanes. Thus, the present invention now makes available for applications requiring contoured structures, the significant advantages of lightweight, structurally efficient, thin skin shell structures with internal stiffening material, all of whose joints have the homogeneity of the parent material. Such lightweight structurally efficient shell structures are highly advantageous when used in applications such as, for example, power conversion equipment, helicopter blades and turbo engine fan blades and vanes. In addition, the present invention achieves its high structural strength to weight ratio within the economic constraints of system cost effectiveness.
The most common method today for fabricating aerodynamic blades and vanes is by forging solid blanks, followed by 100 percent machining to achieve the desired shape and contours. While net precision forgings may also be produced, these require use of special alloys known in the art, but the latter are not as efficient as the wrought alloys. In the case of larger vanes, builtup brazed assemblies are typically produced. Each of these present methods are relatively costly and produce structures which are heavier than desirable. Recent "advances" in the art, such as the filling of hollow structures with suitable potting compounds, have enabled the production of lighter vanes. However, the vanes so produced have suffered from a disappointingly high failure rate because of the strain incompatability of dissimilar materials. A further shortcoming of blades and vanes produced by the methods of the prior art, other than solid structures, are their susceptibility to catastrophic failure caused by foreign object damages.
The present invention overcomes these shortcomings and limitations of the prior art by disclosing an internally stiffened shell structure having lighter weight, lower cost and greater structural integrity than has heretofore been attainable, and a practical method for its fabrication. The phrase "structural integrity", as used herein, relates to the strength and stiffness of the structure per pound and its resistance to catastrophic failure from foreign object damage. The resistance of the present invention to foreign object damage is attributable to the parent material homogeneity achieved by the diffusion bonded joining techniques disclosed herein. The value of this invention is best illustrated by reference to turbofan engine fan blades. As indicated above, these blades are now typically machined from solid material. Hollow, internally stiffened shell structured blades made of the same material by the present invention would have approximately one-third of the weight of solid blades made by the methods of the prior art. Weight saved on a rotating fan blade results in additional weight and economic savings in the full engine configuration by virtue of the reduction of the loads on the fan disc, main shaft, bearings, support structures and containment shrouding. The weight saving multiplier in a typical turbofan engine is in the range of 3-5. Thus, for each pound of weight saved on a fan blade, 3 to 5 pounds of weight is saved in the totally configured engine.
The use of shell structures with internal stiffening material for the manufacture of aerodynamic blades necessarily requires the geometric placement of transition material, in an optimum location, for efficient load transfer from the relatively thin shell material to the more massive root fitting. Ideally, the blade and the root fitting should be a completely homogeneous material. One approach to this ideal structure is to carve the most efficient structural configuration from a mass of homogeneous material having a high strength to weight ratio, thereby precluding any need for joining. However, except for the most simple structural components, the one-piece homogeneous structure is neither weight efficient nor economically feasible. The techniques for producing homogeneous material structural components include machining from bar or plate stock, net forging, forging plus machining and extruding (for constant section members). These production techniques are not applicable to the fabrication of thin wall and hollow sections with internal stiffening material. In addition, the cost of machining aerodynamic thin shapes with varying thickness, camber and twist from solid stock is very high.
Because of the limitations on producing one-piece homogeneous aerodynamic stiffened shell structures, techniques for joining the blade to the root fitting must be used. All typical production methods of joining metals, such as by riveting, bolting, welding, brazing, organic bonding and polyimide bonding result in a load transfer capability lower than that of the parent material utilized; i.e., they do not achieve the required homogeneous properties and strain rate of the parent material across the joint. The solid state diffusion bonding technique, on the other hand, provides (i) a means for achieving full parent material strength across the joint interface because no foreign material is utilized; and (ii) strain compatibility across the joint interface. Several diffusion bonding techniques have been developed, such as, for example, roll bonding, press bonding and vacuum bag bonding. However, each of these techniques imposes limitations on the structural configurations achievable; for example, roll, press and vacuum bag diffusion bonding techniques cannot produce the required blending or filleting. In addition, they are not applicable to complex aerodynamic shapes. Each of these known techniques of diffusion bonding and their respective limitations and shortcomings are briefly described hereinbelow.
The roll diffusion bonding technique utiilizes a steel tooling retort with positioning filler tooling to locate the members to be joined in proper respective positions. The intimate contact is established by roll reducing the retort/tooling and to-be-joined parts by a sufficient percentage (generally 50 to 60 percent) to guarantee completely intimate surface contact and diffusion bonding. This process utilizes expendable tooling and is basically limited to the attachment of members in the rolling direction. The degree of joined member filleting is limited by the combination of tooling material flow and detail parts flow.
The press diffusion bonding technique utilizes reusable positioning and restraining tooling and massive hydraulic presses as the pressure source to establish the intimate contact. However, to utilize reusable tooling, the local surface deformation is generally limited to less than 5 percent. This requires the surfaces to be joined to be matched within very close tolerances. In addition, it also requires, because of the relatively low local unit pressure, a long time at an elevated temperature to allow the diffusion cycle to complete. Flow filleting is very limited because of the low local deformations allowable.
The vacuum bag bonding technique utilizes atmospheric pressure as the pressure source and is therefore limited to very thin sheet structures which can attain the required intimate surface contact at this relatively low pressure. Because of this low pressure, the time at an elevated temperature required to complete the diffusion cycle is very long. In addition, flow filleting cannot be achieved.
As a result of the above-described limitations in the techniques of joining metals, prior art turbofan engine fan blades, for example, are either completely machined from wrought bar stock or forged and completely machined, notwithstanding the high cost of such methods and the lower strength to weight ratio of the resulting blade as compared to that attainable with hollow internally stiffened shell structures. The present invention, however, overcomes these limitations of the prior art and discloses an aerodynamic blade or vane comprised of an internally stiffened shell structure which is diffusion bonded to a more massive root fitting, and a practical method for its fabrication. The invented structure may also include a shroud fitting and/or a tip rib or fitting as required. The invented techniques for diffusion bonding the shell structure to the required fittings achieves filleting and attachment in multiple directions. It also enables one to shape the member intersections so as to minimize stress concentrations. Further, the advantages of the present invention are attainable within the constraints of economic feasibility.